A host of scenarios could have resulted in the loss of an airliner without any distress call being made, including, however unlikely, the complete structural failure of the aircraft’s fuselage.

There is also a remote possibility that the aircraft was struck by a stray missile as part of a military exercise gone wrong. After all, the US was responsible for downing an Iran Air flight in the Persian Gulf in 1988.

However, no one should imagine that, in our security-conscious era, terrorism is an unlikely explanation.

Chinese students stand by candles while praying for the passengers aboard the missing Boeing 777 at a school in Zhuji city. The disappearance may be down to a terrorist act

A group of Malaysian residents pose after lighting candles during a vigil for missing Malaysia Airlines passengers at the Independence Square in Kuala Lumpur

Many people believe that the post-9/11 use of locked and reinforced cockpit doors prevents any hijacker from gaining access to the flight deck, but the reality is rather different.

Complacency over cockpit door procedures is commonplace, and tests have shown that it takes only seconds for a determined person to gain entry when the door is being opened to allow the serving of meals or beverages to the pilots, or to allow them to exit the flight deck to use the toilets.

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We already know that today’s terrorists are often capable of flying aircraft, so the possibility of fanatics gaining entry and neutralising the crew, before piloting the aircraft themselves – potentially as a suicidal mission – needs to be considered.

There have also been cases of crew members taking the controls to commit suicide in the most dramatic fashion. On November 29 last year, the pilot of a Mozambique Airlines flight crashed his Embraer aircraft in Namibia, killing all 33 people on board.

Members of the Republic of Singapore Air Force (RSAF) scan the seas about 140 nautical miles north-east of Kota Baru, Malaysia, for any signs of the missing Malaysia Airlines plane

Oil slicks can be seen from the Republic of Singapore Air Force (RSAF) plane scanning the seas about 140 nautical miles north-east of Kota Baru, Malaysia

The Civil Aviation Authority of Vietnam said on its website searches were being conducted about 140 km (90 miles) southwest of Tho Chu island, which is located about 200 km off the coast of southern Vietnam

The possibility of a further two stolen passports used on the same flight is now being investigated after it emerged that no cross checks were carried out against Interpol's lost and stolen database

The aviation industry is particularly concerned about the ‘insider threat’ – the possibility of a ‘trusted’ employee with access to aircraft smuggling an improvised explosive device on board an aircraft. Such a possibility exists in almost every airport in the world.

As far back as April 11, 1955, it is believed that an airport cleaner at Hong Kong International Airport managed to place a bomb on an Air India flight to Jakarta in the anticipation that Chinese premier Zhou Enlai would be on board.

Zhou did not travel, but the aircraft was destroyed. Nowadays, on a seemingly weekly basis, criminal gangs or illegal immigrants are found to be working at airports in catering, cleaning and maintenance roles.

And, of course, the possibility still exists that a bomb could be secreted on to a flight, either intentionally or unwittingly, in checked luggage or cargo.

Vietnamese military personnel prepare a helicopter for a search and rescue mission for the missing Malaysia Airlines flight

A minister today said rescue helicopters had been scrambled to check a 'yellow object' that rescue teams suspected could have been a life raft from the missing plane, but turned out to be a false alarm

The Singaporean submarine support and rescue vessel, MV Swift Rescue, is prepared before it departs to assist in the search for missing Malaysian Airlines flight MH370 in Singapore

Search and rescue planes scoured waters off the southern tip of Vietnam on Monday, searching for any trace of a Malaysia Airlines jetl 48 hours after it vanished from radar screens with 239 people on board

The security controls, such as X-ray, have never really proven themselves outside of the laboratory. We know, as demonstrated by the destruction of both Pan Am flight 103 over Lockerbie in 1988 and Air India flight 182 in Irish airspace in 1985, that when a bomb explodes on board, with the blast penetrating the fuselage, the rest of the aircraft can literally peel apart in seconds.

So if the loss of MH370 this week was an act of terrorism, who might be behind it?

Possibly the East Turkestan Islamic Movement (ETIM), which seeks independence from China for Xinjiang Province, inhabited by members of the mainly Muslim Uighur ethnic group.

ETIM has become increasingly militant. It is believed it was behind the March 1 attack at a train station in Kunming by a gang of knifemen who stabbed passengers. Twenty-nine people were killed.Worryingly, ETIM is no stranger to targeting civil aviation.

A cabin crew of Division 918 of the Vietnam Air Force is onboard a flying Soviet-made AN-26 during the search operations

A vessel is seen from a flying Soviet-made AN-26 of the Vietnam Air Force during a search operation for the missing Malaysia Airlines Boeing 777 over the South China Sea

Ships are seen from a Soviet-made AN-26 of the Vietnam Air Force during a search operation for the missing Malaysia Airlines plane

An iPad is used to support orientation by a military official inside a Vietnamese Air Force plane during search and rescue operations for the missing Malaysian Airlines flight

On March 7, 2008 (almost six years to the day before the loss of MH370), in the run-up to the Beijing Olympics, a 19-year-old woman called Guzalinur Turdi allegedly attempted to destroy a China Southern flight en route from Urumqi, in Xinjiang Province, to Beijing.

Her suicidal act involved igniting gasoline which she had managed to get on to the aircraft in drinks cans. It was only the actions of the crew, detecting the strange odour, which prevented disaster.

Could it be that ETIM, having failed to gain international publicity through its domestic attacks, has decided to go international?

Although an Islamic state, Malaysia was recently denounced by members of the international community for deporting back to China six ethnic Uighur asylum-seekers.

So the targeting of a Malaysian flight operating to China could be seen as an attack against China and an act of retribution against Malaysia.

Philip Baum is the editor of Aviation Security International and managing director of the security consultancy Green Light Ltd.